1. Field of the Invention
The present invention pertains to the filtering of airborne overspray from an airflow.
2. Description of the Prior Art
Spray booths have long been used in many industrial applications to provide significant advantages in maintaining health and safety standards. In general, without spray booths, workers spraying paints, chemicals, or other materials onto a workpiece would be subject to overspray which is directed toward the workpiece, but which does not stay in contact therewith. One type of spray booth for removing such overspray typically has a ventilation system for establishing an airflow which induces the overspray to travel across one or more filtering stages.
U.S. Pat. No. 4,484,513, assigned to the assignee of the present invention, for example, discloses an improved spray booth having multiple water-wash stages. A first water-wash stage consists of vertically oriented metal baffles having very shallow flanges pointing in a direction opposite to the direction of the overspray. The shape of the baffles provides advantages in maintaining a water flow traveling across the flat, front surfaces of the baffles, i.e., those surfaces extending toward the overspray. Many of the airborne paint particles are trapped by the water flowing down the baffles and carried down the baffles to an underlying tank. Because these baffles are the first wash, a large amount of paint particles are trapped and carried into the tank down below the baffles. Chemical coagulants are added to the water to coagulate the paint particles collected in the water reservoir into a scum that floats and is removed by skimming techniques. The baffles are very effective as a first filtering stage, it has been found that the baffles over a period of time accumulate significant concentrations of overspray material, particularly on the edges of the baffles and extending away from the direction of overspray. This requires that periodic maintenance be performed on the baffles to wipe the accumulated paint from the metal baffles. The paint is a sticky material that does not readily wipe off. The amount of paint sludge accumulating in the tank is quite large and it must be removed either by shutting down the system and scooping out the paint or by sludge removal devices. It is, of course, desirable to reduce the amount of maintenance required to achieve high levels of filtering efficiency. The water in the tank also must eventually be disposed of and it is contaminated water because of the chemicals therein and this water is not easily disposed of. Thus, there is a need for an efficient manner to assist in removing paint spray in such water wash booths.
The coagulant chemicals added to the water add to the yearly cost of running a spray booth as does the cost of cleaning the booth and its down time during cleaning. The amount of sludge collected is another problem since it may be hazardous waste which must be incinerated because of the toxic solvents retained in the paint sludge. Disposal costs for this collected paint sludge are normally about $300 per barrel.
Another problem exists with high solids paint that does not cure or become a dry solid until it is baked and the solvent is driven off. High solid paint, if emitted, will fall on automobiles or other surfaces and, because it is still wet, it can bake on an automotive engine hood when the engine heats the hood and is exposed to sunlight. High solids paint is also expensive and the recovery of overspray for reuse is also desirable.
There is also a need to increase booth efficiency and the present invention is directed toward providing increased both efficiency by providing a pre-baffle system which can collect approximately 80% of the overspray before the overspray reaches the usual filtering stages in the booth. The preferred pre-baffle system only reduces the air flow by about 1/16 or less so that the air flow requirements can still be maintained.
Another advantage of the pre-baffle system is that it can be made of disposable baffles which can be replaced, for example, on a weekly basis and which can be baked to remove the solvents from the paint sludge thereon to dry completely the paint on the baffles which then can be disposed of as non-hazardous waste. By collecting paint particles on the pre-baffles, the amount of overspray impinging on the normal filter stages may be reduced to 20% so that the amount of sludge in the water and the amount of paint collected on the baffles or other filter elements in the booth is drastically reduced. Less cleaning, less water and less sludge on the booth filter stages results in substantial savings. When the filter stages are dry fibrous filter panels as in dry booth, the collection of 80% of the overspray on the pre-baffle system means that the dry filter panels need be changed only once in five times of the current changes. Also, improved air flow may be found in dry booths for longer periods of time because the pre-baffle system helps distribute the air more uniformly across the dry filter panel face. Without the pre-baffle system, the air tends to be more concentrated at the center of the panel face which first fills with paint and then the air and particles are forced outside of the center. This redirection of air in these dry filter booths causes air turbulence and adversely effects the spray pattern of paint being applied to an article in the booth. However, with the pre-baffle system the uniformity of the air pattern is improved in a dry booth to reduce the disturbance of the paint spray pattern.
Because the pre-baffle system can collect 80% of the overspray, it has been found desirable to collect high solid paint overspray for re-use. The high cost of high solid paint and its retention of its liquid state makes it worthwhile to do so. The preferred baffles are made of inexpensive material and, after baking the high solid paints thereon, these disposable pre-baffles may be economically incinerated or lawfully used in a landfill.